The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio when an accelerator pedal of the vehicle is quickly depressed while the transmission ratio is changing.
A known control system for a continuosly variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is decided by line pressure, pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio. In such a system, the speed (rate) of changing of the transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the transmission ratio occurs, which causes the driveability of the vehicle to reduce.
EP-A-No. 207603 shows a system which operates to vary a desired transmission ratio in accordance with the opening degree of a throttle valve and speed of the driven pulley. The transmission ratio changing speed is controlled so that the actual transmission ratio converges to the desired transmission ratio.
However, the actual transmission ratio does not always comply with the calculated desired ratio. For example, the speed of the convergence depends much on the running condition of the belt engaging with the drive and driven pulleys, that is, on whether the diametrical position of the belt relative to the pulleys is stable at a steady state or changing in the diametrical direction at transient state, for changing the transmission ratio. When the transmission ratio is constant, forces of the drive and driven pulleys exerted on the belt are balanced. In order to change the transmission ratio in accordance with depression or release of the accelerator pedal, oil must be applied to or drained from the servo device of the drive pulley, thereby changing the forces of the pulleys acting on the belt. Thus, the belt moves toward or away from the center of each pulley in the radial direction thereof.
Referring to FIG. 6, when the throttle valve is quickly opened from a steady state, the desired transmission ratio (id) rises quickly as illustrated. However, the actual transmission ratio (ia) changes with a delay after rising of the desired transmission ratio, since the movement of the belt is retarded by the grip of the pulleys.
On the other hand, if the accelerator pedal is quickly depressed while the transmission ratio is changing, and hence the belt is moving, the belt moves fast in the diametrical direction. Accordingly, as shown by the dotted line in FIG. 6, the transmission ratio changes rapidly in accordance with the desired transmission ratio id, which causes a shock due to a large change of torque.